The forecaster said the storms had the potential to bring severe weather with damaging, powerful wind gusts, heavy rain and even small tornadoes.

A line of squally thunderstorms struck the Auckland region this morning bringing intense rain.

Each squall burst was expected to bring damaging winds and brief but heavy rain.

Viva the dog enjoys the flooding at Walmsley Park in Mt Roskill after a heavy downpour. Photo / Michael Craig

The thunderstorms were expected to clear northern New Zealand by 2pm but MetService warned it was going to get worse for the rest of the island and top of the South Island as the day wore on.

In a wide ranging forecast of miserable weather engulfing the entire North Island gales were expected to develop everywhere, and even reaching severe strength in some areas. Gales were also expected to rise in Southland and Otago but die back by lunchtime.

Large swells were expected to swamp the entire western coastline of both islands.

A warning was also in force for heavy rain for the Tararua Range with the central North Island high country on a lesser rain watch.

MetService said there was a possibility the heavy rain could reach warning levels in the hills from Wanganui and Waitomo across to Mt Ruapehu this morning and afternoon.

Mid-morning ferry sailings across Wellington Harbour were cancelled as conditions deteriorated in the capital.

Torrential rain had flooded a section of the Haywoods Hill Rd with police advising motorists to take care if travelling on the highway. While the route remained open police warned the road would be slippery from the intense rain.

Heavy squally showers with thunderstorms were set increase across from Waikato to Kapiti and the Marlborough Sounds later this afternoon.

Road snowfall warnings were also in place for the Desert Rd, major passes across the Southern Alps, as well the Dunedin to Waitati Highway as conditions deteriorated throughout the day. As much as 20cm was expected to fall on Arthur's Pass overnight.

An earlier severe thunderstorm warning for Taranaki and the King Country has been lifted.

The Metservice said the squally storms could see winds whipped up to 110 km/h. This could cause some structural damage, including trees and power lines, and make driving hazardous. If any tornadoes occurred they would only affect very localised areas.

There was also a concern over possible surface and flash flooding and slips.

Driving conditions were also expected to be hazardous with surface flooding and poor visibility in heavy rain.